Agricultural combines or “combine harvesters” are well-known for harvesting crops such as corn, soybeans, and wheat. The typical combine includes a self-propelled chassis supported on the ground via driving and driven wheels. A replaceable harvesting head is mounted on the front of the chassis for harvesting the crop of interest. The combine is operable to feed the harvested grain from the head to an internal threshing and separating system that separates the grain from stalks, pods, cobs, etc. (collectively referred to herein as “chaff”) and that transfers the grain to an on-board storage hopper. The stored grain can be periodically transferred to a wagon or the like by an auger mounted on the chassis adjacent the storage hopper.
The threshing and separating system of the typical combine includes at least one threshing rotor, a concave, a grain pan, sieves and fans. Of these components, the rotor is of the most importance for purposes of the present invention. (The rotor(s) will hereafter be referred to in the singular for the sake of convenience, it being understood that the problems addressed by the invention, and the invention itself, are equally applicable to single rotor and multiple rotor systems). Torque is typically transferred to the rotor directly from the engine by a belt drive system that is engaged by a mechanical clutch. However, in order to increase the amount of crop processed by the combine, the size, weight and power consumption of the rotor are being increased to levels above the tolerances of belt driven technology. It is difficult to accelerate such a rotor from rest, particularly under certain crop conditions, because accelerating the high-inertia rotor places high stresses on both the belt drive and the clutch used to engage the belt drive. The loads imposed on the rotor after it is accelerated up to speed also can vary dramatically. The stress on the clutch and belt can be severe, resulting in early clutch and belt failure. Additionally, there are instances in which the combine encounters a “slug” condition in which the operator may determine that the crop is lodged between the rotor and concave. It may be desirable in this situation to permit the operator to control the rotor to reverse the direction of rotor rotation to deslug the rotor.
So-called “split-torque” or “hydro-mechanical” transmissions have been proposed to address these and other problems encountered when driving a threshing rotor. For instance, U.S. Pat. No. 5,865,700 to Horsch discloses a hydro-mechanical drive system including an engine and a hydrostatic motor which derives its power from the engine. A single clutch controls the input of the engine power and input of the hydrostatic motor power. As another example, U.S. Pat. No. 6,247,695 to Hansen discloses a combine in which an engine drives a wet clutch and a hydrostatic motor. U.S. Pat. Nos. 6,695,693 and 6,702,666 to Ho, Brome, and Bundy disclose a combine with a dual path drive system in which an engine drives the rotor through two paths: a hydraulic pump/motor path and a direct gear train path.
In the latter two patents, the two paths are joined at a planetary gear box, with the engine coupled to and directly driving a ring gear, and the hydraulic motor coupled to a sun gear. The output planetary gears are coupled to the rotor through a gearbox. A microprocessor controls the speed of the rotor primarily by regulating the speed of the hydraulic motor driving the sun gear. In the preferred arrangement, the engine runs at a constant, optimum speed at which it is most efficient. An electronic controller connected to a rotor speed sensor is configured to vary the speed of the hydraulic motor until the rotor is operating at its optimum speed as well.
While the addition of a motor to permit gradual acceleration does reduce damage to a belt, it also adds an additional components—the hydraulic motor—that itself is susceptible to damage under extreme operating conditions.
When a split torque system is subjected to severe and sudden loading, the driving torque applied to the rotor shaft by the engine and the motor rises extremely fast and extremely high as the rotor resists further rotation. This sudden increase in torque causes a corresponding increase in hydraulic pressure in the hydraulic lines providing the motor with hydraulic fluid. If this increase in hydraulic pressure is great enough, it will cause the pressure relief valve coupled to the hydraulic lines to open and the motor to overspeed.
Once the pressure reaches this threshold and the hydraulic conduits open, the motor can be accelerated by the applied torque to speeds outside its normal operating range, speeds that may damage the motor itself.
For example, a typical hydraulic motor driving a combine rotor operates at speeds of about +4000 to −4000 rpm. These motors are typically damaged when their speeds reach 5000-6000 rpm, for example. When a rotor is slugged, the sudden increase in engine torque applied to the motor shaft can open the pressure relief valve and accelerate the motor to speeds of 7000-10,000 rpm in the space of just a few seconds. When extreme motor overspeed occurs, the operator must immediately stop harvesting with the combine, and have the motor inspected, overhauled and/or replaced as necessary. This inspection and repair process can take days. Farmers will not tolerate a combine that is broken down for days during the harvesting season.
One way of preventing motor overspeed is to electronically monitor a motor speed sensor and disengage the motor from its load when it reaches an overspeed limit that is below a motor-damaging speed.
One difficulty with this solution is that the speed signal provided by the motor speed sensor has a significant noise component. It can vary substantially from sensor reading to sensor reading, sometimes indicating a speed that is higher than the true motor speed and sometimes indicating a speed that is lower than the true motor speed.
Disengaging the motor based on a signal from the speed sensor may generate too many false positives and false negatives. A “false-positive” is when the sensor indicates the motor is over speeding and it is not. A “false negative” is when the motor speed sensor does not indicate the motor is over speeding and it is. False positives are problem because of the delay in harvesting. When the engine is disconnected and the rotor is shut, the operator must immediately stop the combine, climb out, and inspect the drive system to see whether the rotor is slugged. This takes time. False negatives are problem because of the damage to the motor. If the system does not sense the motor overspeed condition, the motor can be damaged. This, too, delays harvesting.
What is needed, therefore, is an improved method and apparatus for detecting motor speed that decreases the false positives and the false negatives. It is an object of this invention to provide such a method and apparatus.